The pressure sensitive adhesive used for protecting film for polarizer serves pressure sensitive adhesive function between polarizer and polarizer protecting film which protects polarizer from outer shock, friction and contamination in the processes of polarizer preparation to LCD module preparation.
Generally, acrylic pressure sensitive adhesive has been used for surface protecting film for plastics because of its weatherability, transparency, and easiness of control of pressure sensitive adhesiveness. Acrylic pressure sensitive adhesive used now is acrylic copolymers that are copolymerized with monomer having functional group such as carboxy, hydroxyl or epoxy and (meth)acrylic acid alkyl ester, and are crosslinked to poly isocyannate, melanin resin, epoxy resin, or metal chelate.
The pressure sensitive adhesiveness control according to the peeling rate is very important in polarizer protecting film. In case of preparing polarizer, proper low rate peeling strength is required to function as protecting film. If the peeling strength is too low, when heating after the protecting film is adhered, the protecting film is easily loosed due to moisture on a surface of a product adhered thereto, bubbles from inside the product, or shrinking of polarizing film. If the low rate peeling strength is weak, the protecting film is easily loosed at a cutting side, and vulnerable for outer contamination.
In addition, in case of peeling the protection film when inspecting foreign substances attached to a polarizer, the wettability should be excellent so that the protective film can be quickly wet to a polarizer again.
Therefore, protective film for polarizer should have proper characteristics so as to have appropriate low rate peeling strength and high rate peeling strength, and good wettability.
Generally, to make an adhesive for protective film less sensitive to change of pressure sensitive adhesiveness according to the peeling rate, the crosslinking degree of adhesive for protective film is set very high. But, in acrylic adhesive, if the adhesiveness according to the peeling rate is simply controlled by crosslinking degree, slow peeling ability and fast peeling ability show different results from each other. In other words, if the crosslinking degree is reduced, the pressure sensitive adhesiveness, especially after heating, will be stronger, and the peeling strength will be highly increased at high rate peeling. If the crosslinking degree is increased, the low rate peeling strength as well as high rate peeling strength will be decreased, and the wettability will be deteriorated.
Japan Patent Laid-open Publication No. 5-163468 and 11-256111 describes a method of reducing high rate peeling strength by preparation of pressure sensitive adhesive having more than 60% of gel content by adding epoxy crosslinking agent to 100 parts by weight of acrylic copolymers containing carboxy group. But, this method has a problem that low rate peeling strength and high rate peeling strength are too high.
Japan Patent Laid-open Publication No. 2001-323239 tried to decrease high rate peeling strength by copolymerizing acrylic acid ester having 3˜10 carbon numbers with 5˜60 parts by weight of acrylic acid ester having more than 12 carbon numbers of alkyl group. But, in this case, the glass transition temperature was increased to give bad effect to wettability by the long-chain alkyl group having more than 12 carbon numbers, and the adhesiveness with substrate was reduced.
Japan Patent Laid-open Publication No. 200341229 tried to improve wettability and repeeling property by using acrylic copolymer having less than 15 of molecular weight distribution and more than 300,000 of weight average molecular weight, and specific 2-functional isocyanate crosslinking agent. But, no technical idea about high rate peeling strength was mentioned therein.
Further, Japan Patent Laid-open Publication No. 2005-23143 disclosed the technical features to set more than 90% of crosslinking degree to satisfy peeling strength of protective film. But, it has a problem that such high crosslinking degree reduced low rate peeling strength as well as high rate peeling strength, and so the protective film had a problem that the basic function of protective film, i.e., protecting the panel from outer contamination was difficult to be fulfilled.
Therefore, to satisfy wettability and high rate peeling strength, a low molecular weight plasticizer could be added to pressure sensitive adhesive having high crosslinking degree. But, the basic function of surface protection was difficult to be served since the low rate peeling strength is rapidly lowered by adding a small amount of plasticizer. In addition, recently, to improve the surface property of polarizer, LR (low reflectance) surface treated or AR (anti reflectance) surface treated polarizer has been marketed. But, they have a problem that it was easy to make the unique optical property of polarizer bad since the plasticizer of pressure sensitive adhesive layer was transferred onto the polarizer at the time of peeling protective film, by using the polarizer protective film having a lot of plasticizer.
Thus, there has been a continuous need to develop a pressure sensitive adhesive composition for protective film, with maintaining suitable low rate peeling strength, having good repeeling strength by low rate peeling strength at high speed, and having excellent wettability for an adhered product, at the same time of minimizing addition of a plasticizer.